Quad motor vs locking diff

[MountainBikeDude]

I have come to the conclusion that the people in this conversation fall into 2 categories;

1. Those who understand the concept and importance of "rolling friction".
2. Those who don't.

If you don't, you don't. You will not be up to speed in the conversation untill you do.

For anyone wishing to get up to speed ?

https://en.m.wikipedia.org/wiki/Rolling_resistance

Sponsored

 

[R.I.P.]

For anyone wishing to get up to speed ?

https://en.m.wikipedia.org/wiki/Rolling_resistance

Edit: hum, actually that is rolling resistance, not rolling friction. Actually even though they are cross referenced in that link, they are completely different concepts.

I am quite sure some of you can explain this better than I can, so feel free to chime in but here goes:

Rolling Friction:
Although I am sure there is a more appropriate scientific term for it, we enthusiast types use this term to refer to the lateral resistance a rolling wheel retains as long as it is moving.

A static wheel has static friction, which is greater than sliding friction.

A freely rolling wheel maintains it's "lateral static coefficient" even when it is rolling; up until the friction is broken.

Once broken, the wheel will move in whatever direction forces act upon it, regardless of which direction it happens to be turning.

This is why a braking car on ice is out of control until the driver lets up on the brakes and lets at least 2 of the wheels spin with no resistance what-so-ever. If you can get at least a front & rear wheel "connected" and spinning again in this way, you will regain as much lateral control as the surface will allow. The other two can continue trying to slow (or accelerate) the vehicle while the "anchor" wheels maintain lateral control (i.e. keep you from sliding over the edge).

How did I do, is that close?

This is slick surface driving 101. NO! You do NOT want to use any more brake than you absolutely have to. NO!! You do NOT want your diff in any kind of locked or LS condition! You MUST have your diffs open, to allow complete free-wheeling of the wheels with the most traction to keep your vehicle from spinning out, moving sideways or falling off the trail. If you cannot gain enough traction with your other 2 wheels, you will be looking for other options (winch), but unless you are that nuub that just bought a Jeep and wanted to tag along with the big boys, you do NOT lock your diffs; or over the side you go.

I know I did not do as well at explaining this as some of you may be able to do, but I have come to see that there are those not aware of how this works.

 

[graemebshaw]

Don't look now, but there are multiple topics being discussed throughout this thread. What you are talking about may not be specifically what I'm talking about. That said, I've referenced Kyle's video several times already in my posts, so if you are initially going to reply to my comment, then the burden of understanding what I'm referencing is really on you.

So again, do you disagree that in Kyle's video that having the quad motors behave more like a limited slip diff would be more beneficial than behaving like an open diff (which is actually what they are doing in the video) to have helped him drive up that hill? Or are we not even agreeing on that?

Of course, an open diff would not help Kyle in his video and lockers would help. That was never in doubt. I think everyone’s frustration with your posts stem from the fact that we moved on from that issue 10 pages ago.

But FWIW, later on (maybe further up the hill) I recall Kyle mentions that the stationary wheels were actually being driven with max available torque but were still unable to pull the vehicle up and over the obstacle. So his Rivian was not exactly acting like it had an Open diff, but rather like a LSD with insufficient torque. I have no means of checking if that is the case, but that was the calculation that an earlier poster was trying to confirm.

But in any case, most of us are more concerned with the challenges of replicating an open diff in certain low-mu scenarios.

 

[Ralph]

It's 100% controlled by software now. You're reading too much into my words. I'm saying that it would be easy to change (does that work better for you than the word "fix"?) the software from it's current state to a condition where it will provide more torque to the wheels tnat have more traction.

I'm reading the words you wrote. And write again above.

First you don't *know* that "it would be easy to "change" - or fix.

Secondly, you don't have any idea what the implications *might* be to make the "easy" change in software - in context of the problem as it is defined by the vehicle - as it sits. But you continue to ignore this as it relates to the multiple issues at hand. Preferring to (now) cherry pick the problem so that you can insist on the solution you *believe* , perhaps correctly to be "easy".

And, as you have now been told by several others (again), the very real problem depicted in Kyle's video is only one of issue that the current implementation of software/hardware does not adequately address - for some people, in some situations.

The bottom line is , regardless of the language used, you have implied over and over again that the problem is trivial and up until now could be simply "fixed" with a software tweak. Now the word is "changed" and applies to only one of the problems.

Tomorrow the word will be "improved". And my guess Rivian will indeed improve on these and other issues. By enough? Who knows. I for one sincerely hope so. But it *may* not be trivial.

Just because tech has advanced very quickly over your lifetime does not necessarily mean the environment of the truck and the tech that can be readily applied will solve the issues present. "Past performance does not guarantee future results."

This whole discussion reminds me of an old, tired joke about the most genius invention. In variations many great things are proposed. Then someone suggests the "thermos" (yesterday's yeti cup). The reason given? "It keeps cold things cold and hot things hot. How does it know?"

Physics at work....

 

[MountainBikeDude]

Edit: hum, actually that is rolling resistance, not rolling friction. Actually even though they are cross referenced in that link, they are completely different concepts.

I am quite sure some of you can explain this better than I can, so feel free to chime in but here goes:

Rolling Friction:
Although I am sure there is a more appropriate scientific term for it, we enthusiast types use this term to refer to the lateral resistance a rolling wheel retains as long as it is moving.

A static wheel has static friction, which is greater than sliding friction.

A freely rolling wheel maintains it's "lateral static coefficient" even when it is rolling; up until the friction is broken.

Once broken, the wheel will move in whatever direction forces act upon it, regardless of which direction it happens to be turning.

This is why a braking car on ice is out of control until the driver lets up on the brakes and lets at least 2 of the wheels spin with no resistance what-so-ever. If you can get at least a front & rear wheel "connected" and spinning again in this way, you will regain as much lateral control as the surface will allow. The other two can continue trying to slow (or accelerate) the vehicle while the "anchor" wheels maintain lateral control (i.e. keep you from sliding over the edge).

How did I do, is that close?

This is slick surface driving 101. NO! You do NOT want to use any more brake than you absolutely have to. NO!! You do NOT want your diff in any kind of locked or LS condition! You MUST have your diffs open, to allow complete free-wheeling of the wheels with the most traction to keep your vehicle from spinning out, moving sideways or falling off the trail. If you cannot gain enough traction with your other 2 wheels, you will be looking for other options (winch), but unless you are that nuub that just bought a Jeep and wanted to tag along with the big boys, you do NOT lock your diffs; or over the side you go.

I know I did not do as well at explaining this as some of you may be able to do, but I have come to see that there are those not aware of how this works.

Good explanation, I made the mistake of searching "rolling friction" and linking as you said a cross linked article that didn't adequately delve into the difference between the two, just joined them together, and called it a day.

On a side note, I find it very difficult to stay engaged with anything you've posted due to the consistent condescending nature of every one of them. Someone says something you don't agree with, they're a "noob", or "one of those people" or they get dragged into a endless ho hum of how you're right. It's effin irritating to read.

I keep checking in on this thread out of genuine curiosity, to see if anyone can provide some sort of Eureka! analysis or tear down of the situation to come up with a possible solution. Instead, you get a glimmer of something meaningful, that turns into what I mentioned above, or what I'm currently posting, a rant to clog up the thread.

?
?

 

[R.I.P.]

Good explanation, I made the mistake of searching "rolling friction" and linking as you said a cross linked article that didn't adequately delve into the difference between the two, just joined them together, and called it a day.

On a side note, I find it very difficult to stay engaged with anything you've posted due to the consistent condescending nature of every one of them. Someone says something you don't agree with, they're a "noob", or "one of those people" or they get dragged into a endless ho hum of how you're right. It's effin irritating to read.

I keep checking in on this thread out of genuine curiosity, to see if anyone can provide some sort of Eureka! analysis or tear down of the situation to come up with a possible solution. Instead, you get a glimmer of something that turns into what I mentioned above, or what I'm currently posting, a rant to clog up the thread.

?
?

Yeah, I get that. There is a reason I live in the woods away from people. I am confident that you would like me even less in person. I am not PC nor do I care to be.

None-the-less, I do work at trying to soften the hard edges when participating in public forums such as this one.

Some days I do better than others.

 

[graemebshaw]

One thing that Rivian could do (I think) within the existing system envelope is to allow the driver to manually put any or all of the motors in neutral. I could imagine a menu item or smart button allocation that would allow you to cut all power and regen to specific motors of your choosing. It would not be reactive, but might help in many off-roading situations.

 

[Yellow Buddy]

Good review, illustrating very well why quad motors can't really compete with full locking diffs. It is interesting to see in the comments that people believe this can be "fixed" with software updates. I guess it is just a factor of people not understanding the technology.

A traditional four-wheel drive with fully locked diffs turns all four wheels at exactly the same speed. One wheel can never, and this is the point that is so very important NEVER slip. Any slip can mean falling off a rock and impaling your truck. Any slip starts to create holes that you have to dig yourself out of. When it is critical that you don't slip any wheel, four wheels locked into identical rotation speeds can never be approximated by a quad motor or software.

While this creates a handicap the quad motor Rivian can never overcome off-road, it has a similar handicap on road. When conditions become super slippery, (snow and ice is an example), a traditional four wheel drive can lock both front and rear axles together while not locking the differentials. In this mode there are at least two wheels at any given time providing lateral stability while the other two attempt to either increase or decrease speed. Again, four disconnected wheels on motors and software can never really approximate this very important ability.

I doubt that Rivian will ever admit that there is more than just a cost savings motivation behind the new dual motor version. As cool as the quad motors are, and as useful as they are in some circumstances, they really do carry some very basic handicaps.

Thank you Kyle for a decent demonstration of at least the first concept.
??

One additional thing I've been wondering about. Engineering explained did a video on locking diffs but...

The R1T's power is split between 4 motors. That's 200hp/225lb tq per motor. In the event of a slip you end up relying solely on the motors that have traction. 3 wheels=600hp/675lb, 2=400/450, 1=200hp/225lb tq right?

With a locking diff, you get most of the power (*driveline loss?) to a single wheel according to the video.

So if we take a F-150 Lightning with dual motors at 580hp/775lb or 390hp/387lb per motor. When it locks you still get power from both motors. 3 wheels = 580hp/775lb, 2 wheels = 390hp/387lb, 1 wheel = 390hp/387lb.

Which would mean despite the significantly less power it is actually able to put down similar power and in some cases more because of the configuration. I'm not sure if I'm understanding that correctly, and don't off road - just trying to understand.

 

[R.I.P.]

One additional thing I've been wondering about. Engineering explained did a video on locking diffs but...

The R1T's power is split between 4 motors. That's 200hp/225lb tq per motor. In the event of a slip you end up relying solely on the motors that have traction. 3 wheels=600hp/675lb, 2=400/450, 1=200hp/225lb tq right?

With a locking diff, you get most of the power (*driveline loss?) to a single wheel according to the video.

So if we take a F-150 Lightning with dual motors at 580hp/775lb or 390hp/387lb per motor. When it locks you still get power from both motors. 3 wheels = 580hp/775lb, 2 wheels = 390hp/387lb, 1 wheel = 390hp/387lb.

Which would mean despite the significantly less power it is actually able to put down similar power and in some cases more because of the configuration. I'm not sure if I'm understanding that correctly, and don't off road - just trying to understand.

With the Lightning's configuration, you will get 100% of the rear motor's power all of the time whether to one wheel or both, depending on where the traction is when it is locked. In the front (and when the rear is unlocked) the amount of power that gets to the ground will be variable, depending on the traction.

There is no transfer case in between, so only the motors on the individual axles are able to put power to those wheels, you cannot move power front to rear, did I understand your question correctly?

 

[racekarl]

One additional thing I've been wondering about. Engineering explained did a video on locking diffs but...

The R1T's power is split between 4 motors. That's 200hp/225lb tq per motor. In the event of a slip you end up relying solely on the motors that have traction. 3 wheels=600hp/675lb, 2=400/450, 1=200hp/225lb tq right?

With a locking diff, you get most of the power (*driveline loss?) to a single wheel according to the video.

So if we take a F-150 Lightning with dual motors at 580hp/775lb or 390hp/387lb per motor. When it locks you still get power from both motors. 3 wheels = 580hp/775lb, 2 wheels = 390hp/387lb, 1 wheel = 390hp/387lb.

Which would mean despite the significantly less power it is actually able to put down similar power and in some cases more because of the configuration. I'm not sure if I'm understanding that correctly, and don't off road - just trying to understand.

Unfortunately it's not as simple as just dividing the motor's figures by the number of wheels. Power and torque are not affected by differentials in the same way, so in some cases a wheel can get 100% of available torque but less than 100% of available power.

 

[R.I.P.]

One thing that Rivian could do (I think) within the existing system envelope is to allow the driver to manually put any or all of the motors in neutral. I could imagine a menu item or smart button allocation that would allow you to cut all power and regen to specific motors of your choosing. It would not be reactive, but might help in many off-roading situations.

This has certainly been tossed around, but most agree there are drawbacks.

In a 100 foot stretch of road you may bank in opposite ways a couple of times and change sides that have the most traction 50 times. A differential will easily and automatically swap sides it is delivering power to all 50 times. Trying to mimic that manually on the fly would not only require you to actually know where the traction is at the moment (difficult, even impossible), but would be a lot of work at a potentially busy time in the drive.

 

[MaskedRacerX]

None-the-less, I do work at trying to soften the hard edges when participating in public forums such as this one.

Some days I do better than others.

These folks have outstanding reviews, and it looks like it's only a 35 minute drive from you ...

https://www.leafly.com/dispensary-info/canna-king

Might help to chamfer those sharp corners

(Side note: also answering your 4xe question)

 

[Biturbowned]

Unfortunately it's not as simple as just dividing the motor's figures by the number of wheels. Power and torque are not affected by differentials in the same way, so in some cases a wheel can get 100% of available torque but less than 100% of available power.

Can you explain this? I’m not following.

 

[MaskedRacerX]

But on a different note, now do you like your 4xe?

It's solid. If I may derail the thread a bit ...

It was our first EV, after deciding we wanted to give EV tech a shot. Funny enough, my original motivation came from all the friends/associates I knew with EVs (mostly Tesla) but from a Dad's Day gift a cordless, electric blower (to replace a broken corded blower).

Got other Greenworks yard tools without the battery/charger, making them super inexpensive (and I figured with my light use, I could easily share), so I scored a string trimmer and a pole saw, eventually got a mower for, a killer deal came up on Woot (that included TWO batteries and charger). Then something clicked, batteries, no smell, no gas ... I could drive a vehicle like this! YES!

But we were a little apprehensive to go full on BEV. About this time, our previous 2nd car/family/wife's ride, a DD/RT was coming off lease, we decided we'd just try something without 3-rows, we both kind of wanted a Wrangler (around here, every 4th car is a Wrangler ...) We bought the DD/RT off lease, and turned around and sold it to Carvana for a $12K profit

So we got the 4xe, we were a pretty early adopter, got ours in April '21, got the $7500 tax credit immediately as a cap reduction, even a discount (this was pre-multiple-price-hikes).

Once I got the 50a circuit installed, and a 40a EVSE it was a total epiphany. So much so, that I sold my '19 GT convertible and picked up an M3P (they share the Grizzl-E EVSE, for the Tesla, I just pop on the adapter that was supplied with the car). I went with an outlet vs. hardwired, and got an N6-50 adapter for the Tesla mobile connector, so I have an L2 charging backup.

Anyway, back to the 4xe, we LOVE it, I installed an electric hoist in the garage, so removing the top is easy (and it stays up and out of the way), we use it for beach cruising, heading down to the point for a little SUP-ing, to lug shit around, and it's almost exclusively our around town car. It's extra great around here too, where there's constant flooding issues, it just chugs through water no problem. It's so nice quietly cruising around the beach areas, the only sound the ocean and the occasional tires squeaking in the sand.

It runs about 95% of the time on battery only, and yes, it doesn't have a ton of range, but we rarely use it beyond that range (we work from home, daughter's school is ~6 miles away).

So that means in the last 1200 miles or so, it's used less that 1/2 tank of gas.

I probably don't have to tell people on this board, how fun Wranglers can be, all the aftermarket, modular body parts, super durable interior, and heck, ours is a big sissy model with heated seats and a fancy stereo system But it's still very much a Wrangler, a real 4WD system, low range, and when the gas AND electric systems kick in, it's had a f*** ton of power (it's shockingly quick on the road too, for a big brick).

 

[Yellow Buddy]

Can you explain this? I’m not following.

I oversimplified it in my example. But to start we have different sized motors. The type of differential will also matter in terms of power distribution, where it's situation the power distribution, driveline factor losses, etc. That's my understanding of it.

I was simply trying to understand the available power when it comes to standard motor design.

My understanding based on the replies..and I still may be wrong.

Using just the rears the maximum power available on one wheel on a quad motor will be limited to that single motor for that wheel. Vs the maximum power available to one wheel on a dual motor w/ locking diff is still limited to that single motor, less any losses/tuning due to the diff. But that motor may have higher power naturally because it's designed to typically handle two wheels.

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